Steam turbine and method for extracting moisture from a steam turbine

ABSTRACT

A steam turbine is disclosed which includes a casing containing a plurality of expansion stages, each including stator and rotor airfoils. Downstream of the stator airfoils of one or more expansion stages, the casing includes a slot arranged to receive moisture that concentrates on an end wall of the stator airfoils and discharge it to the outside. Each end wall has a side portion facing the slot having a variable radial obstruction in a tangential direction, in order to define with the slot a passage through which the moisture may pass that is smaller or closed at the suction side and larger at the pressure side of the stator blades, to at least partially prevent the dry steam from entering the slot at the suction side of the blade.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to European Patent Application No. 09153874.4 filed in Europe on Feb. 27, 2009, the entire content of which is hereby incorporated by reference in its entirety.

FIELD

The present disclosure relates to a steam turbine and a method for extracting moisture from a steam turbine.

BACKGROUND INFORMATION

Steam turbine plants, such as plants for electricity generation are known to include a steam generator that feeds a turbine. The turbine expands the steam to operate an electrical generator and then discharges the exhausted steam in a condenser.

In order to extract high power from the steam, it is known to expand steam far into the wet steam region.

Even though this allows power to be increased, this may cause the generation of moisture within the steam that may have a number of negative effects.

Because of their high momentum of inertia, moisture droplets, such as droplets which exceed a certain size, cannot follow the turning of the steam flow. Thus, they impinge on the following blade surface causing the so called wet steam erosion.

This can be particularly relevant in low pressure turbines where expansion in the wet steam region takes place and often the amount of moisture may be large.

Steam turbines, such as low pressure stages of steam turbines, can be provided with devices that extract the moisture and discharge it to the outside.

With reference to the stator blades, such devices can be provided upstream of the stator blades and/or downstream of the stator blades.

In the following, the extraction devices downstream of the stator blades will be discussed in detail.

Steam turbines have a casing that carries the stator airfoils.

Each stator airfoil has an end wall that may be connected to the casing and from which a stator blade extends.

Downstream of the stator airfoil, the casing may be provided with a circumferential slot arranged to receive the moisture that during operation concentrates near the end wall of the airfoils (due to the centrifugal forces caused by the swirling steam flow and the rotating parts).

In order to protect the blades from erosion, the amount of moisture that is extracted trough the slot should be as high as possible. Thus, the slot can be as large as possible to let a large amount of moisture be seized therein.

Nevertheless, a large slot (in addition to a large amount of moisture) may also seize a large amount of dry steam.

This may cause a loss in performance.

SUMMARY

A steam turbine includes a casing having plural expansion stages, each including stator and rotor airfoils. A slot is arranged in the casing downstream of the stator air foils of at least one expansion stage for receiving moisture that concentrates on an end wall of the stator airfoils and for discharging the moisture. Each end wall includes a side portion facing the slot, which has a variable radial obstruction in a tangential direction to define, with the slot, a passage for the moisture that is smaller or closed at a suction side of stator blade and larger at a pressure side of the stator blade, to at least partially prevent the dry steam from entering the slot at the suction side.

A method for extracting moisture from a steam turbine includes a casing containing plural expansion stages each including stator and rotor airfoils. The method includes providing, downstream of the stator airfoils of at least one of the plural expansion stages, the casing with a slot arranged to receive moisture that concentrates on an end wall of the stator airfoils; discharging said moisture; and at least partially preventing dry steam from entering the slot at a suction side of a blade of the steam turbine by using any end wall of the stator airfoils.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the disclosure will be more apparent from the description of a preferred but non-exclusive embodiment of the steam turbine and the method according to the disclosure, illustrated by way of non-limiting example in the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of three exemplary stator airfoils according to the disclosure associated with a slot of the casing (the slot is shown in dashed line).

FIGS. 2-4 are schematic cross section through lines II-II, III-III and IV-IV of FIG. 1, wherein a part of the casing defining the slot is also shown.

FIG. 5 is a further schematic prospective view of three exemplary stator airfoils according to the disclosure; wherein the slot is shown in dashed line.

FIG. 5A is a schematic prospective view of three exemplary stator airfoils according to the disclosure in different embodiments; wherein the slot is shown in dashed line.

FIG. 6 is a schematic longitudinal section of a portion of an exemplary turbine according to the disclosure.

FIG. 7 is a perspective view of an exemplary stator airfoil row according to the disclosure.

FIGS. 8 and 9 are exemplary details of FIG. 7, which for sake of clarity show only some of the stator airfoils.

DETAILED DESCRIPTION

A steam turbine and a method are disclosed for extracting moisture from a steam turbine.

A steam turbine and a method as disclosed can let a large amount of moisture be extracted from low pressure expansion stages with respect to known steam turbines, but at the same time that may cause only a small extraction of dry steam.

A steam turbine and a method are also disclosed that can increase both performance and reliability with respect to known steam turbines.

An exemplary steam turbine of FIG. 11 includes a casing 1 that contains a plurality of expansion stages.

Each expansion stage includes a plurality of stator airfoils 2 downstream of which a plurality of rotor airfoils 5 are provided. For example, each stator airfoil 2 includes an end wall 3 that is connected to the casing 1 and from which a stator blade 4 extends.

In order to extract moisture from the steam turbine, downstream of the stator airfoils 2 of one or all the expansion stages (the enclosed figures show only one exemplary stage), the casing 1 can include a circumferential slot 7 that is arranged to receive the moisture that concentrates on the end wall 3 of the stator airfoils 2 and discharge it to the outside of the steam turbine, for example via holes 8.

Each stator airfoil 2 can have the end wall 3 define, with the slot 7, a passage 10 through which the moisture may pass.

This passage 10 can be smaller or closed at the suction side 12 and may be larger at the pressure side 13 of each stator blade 4, to at least partially prevent the dry steam from entering the slot 7 at the suction side 12 of the blades 4.

In order to define a smaller or closed passage 10 at the suction side 12 of the stator blades 4, each end wall 3 may have a portion facing the slot 7 having a variable radial obstruction in a tangential direction.

In this respect, the portion facing the slot 7 of the end wall 3 may have at the suction side 12 a thick zone 15 that is located adjacent to the stator blade 4 and partially or completely obstructs the slot 7.

Moreover, the portion facing the slot 7 of the end wall 3 may have a thin zone 17 that is adjacent to the thick zone 15 in a tangential direction.

Advantageously, the thin zone 17 may have a thickness that, in a tangential direction, continuously decreases from that of the thick zone 15 towards the side of the end wall 3.

The thin zone 17 also may have a thickness that, in an axial direction, continuously decreases from its portion opposite the portion facing the slot 7 toward the portion facing the slot 7.

The operation of the steam turbine of the disclosure will be apparent to those skilled in the art from the description and drawings, and may, for example, be substantially as follows:

Steam flow flows in an axial direction (as shown by arrows F) and, when passing through the stator and rotor blades 4, 5, can turn to follow the curved channels.

Due to centrifugal forces and inertia, the moisture can be forced to concentrate on the end walls 3 of the stator airfoils 2, at the pressure side 13 of their blades 4.

In addition, the particular shape of the end walls 3 can force the moisture to run towards the zone of the end walls 3 at the pressure side 13 of the blades 4 (see arrows M).

From this zone the moisture can pass through the passages 10 and enters the slot 7 to be discharged to the outside of the steam turbine.

In addition to the moisture, dry steam may also be discharged through the passages 10.

Nevertheless, as the zones close to the suction sides 12 of the stator blades 4 are not provided with passages 10 (or, in any case, these passages are smaller than that at the pressure sides 13) neither moisture nor dry steam may be discharged through these zones (or, for example, only limited amounts are discharged).

Because the moisture is concentrated in the zones close to the pressure sides 13 of the blades 4, whereas the dry steam can be almost uniformly distributed in the channels defined between two adjacent stator blades 4, in an exemplary steam turbine as disclosed herein the amount of moisture discharged can be increased and/or the amount of dry steam discharged can be reduced with respect to known steam turbines.

FIG. 5A shows a different exemplary embodiment of the disclosure. In this embodiment the same reference numbers are used for elements already described.

In this embodiment the variable radial obstruction may be defined by a slope at the end wall.

Moreover, between the thick zone 15 and the thin zone 17 there may be provided a step. The step can have a wall 20 perpendicular to the surfaces of the thick zone 15 and thin zone 17 or, alternatively, tilted with respect to them (or at least one of them).

FIGS. 7-9 show a further exemplary embodiment of the disclosure. In this embodiment the same reference numbers are used for elements already described.

In this embodiment, the variable radial obstruction may also be defined by a slope at the end wall. The slope projects inwardly from the leading edge 18 of each stator blade 4 to the trailing edge 19 of the same stator blade 4. Moreover the slope may be adjacent to each stator blade 4 at its suction side 12.

The present disclosure also relates to a method for extracting moisture from a steam turbine that includes a casing 1 containing a plurality of expansion stages each including stator and rotor airfoils. Downstream of the stator airfoils of at least a stage, the casing 1 is provided with a circumferential slot 7 arranged to receive the moisture that concentrates on an end wall 3 of the stator airfoils 2 and discharge it to the outside.

According to an exemplary method of the disclosure, the end wall 3 of the stator airfoils 2 drives the moisture away from the suction side 12 of its blade and at least partially prevents the dry steam from entering the slot 7 at the suction side 12 of the blade 4.

A steam turbine and the method as disclosed herein can be implemented with numerous modifications and variants, all falling within the scope of the inventive concept. Moreover all details can be replaced by technically equivalent elements.

In practice, the materials used and the dimensions can be chosen at will according to a desired application and the state of the art.

Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.

REFERENCE NUMBERS

-   1 casing -   2 stator airfoil -   3 end wall -   4 stator blade -   5 rotor blades -   7 slot -   8 holes -   10 passage -   12 suction side of the blades -   13 pressure side of the blades -   15 thick zone of the end wall -   17 thin zone of the end wall -   18 leading edge of the stator blade -   19 trailing edge of the stator blade -   20 wall of the step -   F steam flow -   M moisture 

1. Steam turbine comprising: a casing having plural expansion stages, each including stator and rotor airfoils, a slot arranged in the casing downstream of the stator air foils of at least one expansion stage for receiving moisture that concentrates on an end wall of the stator airfoils and for discharging the moisture, wherein each end wall includes a side portion facing the slot, which has a variable radial obstruction in a tangential direction to define, with the slot, a passage for the moisture that is smaller or closed at a suction side of stator blade and larger at a pressure side of the stator blade, to at least partially prevent the dry steam from entering the slot at the suction side.
 2. Steam turbine as claimed in claim 1, wherein said slot is a circumferential slot.
 3. Steam turbine as claimed in claim 1, wherein the side portion facing the slot of the end wall comprises: at the suction side, a thick zone located adjacent to the stator blade and at least partially obstructs the slot.
 4. Steam turbine as claimed in claim 3, wherein said thick portion completely obstructs the slot.
 5. Steam turbine as claimed in claim 3, wherein said thick portion is adjacent to each of plural stator blades at the suction side.
 6. Steam turbine as claimed in claim 3, wherein, the side portion facing the slot of the end wall comprises: a thin zone adjacent to the thick zone in a tangential direction.
 7. Steam turbine as claimed in claim 6, wherein, in a tangential direction, said thin zone has a thickness that continuously decreases from that of the thick portion toward the side of the end wall.
 8. Steam turbine as claimed in claim 6, wherein, in an axial direction, the thin zone has a thickness that continuously decreases in a direction from opposite the slot towards the slot.
 9. Steam turbine as claimed in claim 1, wherein the variable radial obstruction comprises a slope at the end wall.
 10. Steam turbine as claimed in claim 6, comprising: a step between the thick zone and the thin zone.
 11. Steam turbine as claimed in claim 10, wherein said step comprises: a wall perpendicular to surfaces of the thick zone and thin zone.
 12. Steam turbine as claimed in claim 10, wherein said step comprises: a wall tilted with respect to surfaces of at least one of the thick zone and thin zone.
 13. Method for extracting moisture from a steam turbine including a casing containing plural expansion stages each including stator and rotor airfoils, the method comprising: providing, downstream of the stator airfoils of at least one of the plural expansion stages, the casing with a slot arranged to receive moisture that concentrates on an end wall of the stator airfoils; discharging said moisture; and at least partially preventing dry steam from entering the slot at a suction side of a blade of the steam turbine by using any end wall of the stator airfoils.
 14. Method as claimed in claim 13, comprising: arranging the end wall of the stator airfoils to drive the moisture away from the suction side. 